The present invention relates to stationary combustion turbines and more particularly to combustors having a tubular stepped ring sidewall structure.
The stepped ring combustor design is commonly used to provide a capability for operating combustor baskets for extended periods of time at higher combustor gas temperatures needed to supply higher turbine inlet gas temperatures at higher engine efficiencies.
In the stepped ring design, the combustor sidewall is formed by rings which are disposed in telescopic relation to each other. The upstream end of each ring overlaps the downstream end of the next adjacent ring and is supported outwardly thereof, typically by an annular web-like support member or band.
An annular slot is thus provided between each pair of rings for admission of an annular film of relatively cool compressor discharge air from the enclosed turbine space about the combustor shell. As the air in the annular film flows downstream, heat is transferred to it from the combustor metallic sidewall and it generally holds the hot combustor gases away from contact with the combustor sidewall. As a result, combustor sidewall temperatures can be maintained below the metallurgical design temperature limit (such as 1500.degree. F.) even though the combustor is producing turbine inlet gas at an elevated temperature such as 2200.degree. F.
The performance of the stepped ring combustor design directly depends on the heat transfer coefficient and the cooling effectiveness of the annular film of air. The cooling effectiveness of the air film in turn depends on how well the film holds together against mixing with the hot internal combustor gases.
In general, it has been determined that an annular corrugated spacer band secured to the downstream end of each combustor sidewall ring and the upstream end of the next downstream sidewall ring is effective in providing the necessary rigidity for the combustor sidewall while providing relatively little obstruction to coolant air flow through the resultant annular space of slots between adjacent sidewall rings.
The spacer or support member used to secure the sidewall rings together provides an annular slot therebetween and obstructs the coolant air flow through the annular slot to a small extent. In the case of the annular corrugated spacer member, the total flow obstructing cross-section of the corrugation spacer walls is a relatively small percentage of the total cross-section of the annular slot.
As a consequence of the presence of the corrugated spacer walls, coolant air flow divides into stream flows within the respective corrugated spacer chambers within the annular slot. The stream flows rejoin as they emerge from the corrugated spacer with wakes existing between the stream flows for a limited distance downstream to the point where uniform annular flow may be resumed.
To discourage early mixing of the hot internal combustor gases with the coolant air film, an extended lip structure has been provided on the downstream end of each upstream sidewall ring. The lip forms an extension of the sidewall ring and thereby widens the annular slot in the downstream direction beyond the downstream end of the corrugated spacer. Extended lip structure of this kind is shown and described in U.S. Pat. No. 3,307,354 issued to R. W. McCauley, et al. on Mar. 7, 1967 and in a patent application U.S. Ser. No. 679,292 entitled Gas Turbine Combustion Chamber filed by S. S. Osborn, et al. on Apr. 22, 1976, assigned to the present assignee and now abandoned. Since the corrugated spacer band has relatively small obstruction cross section, the velocity of the annular coolant flow experiences little dropoff as it passes from the corrugated space member into the annular slot space defined by the extended lip.
With the extended lip structure, sufficient distance exists in the annular slot downstream of the corrugated spacer to permit dissipation of the wakes and substantial reformation of a uniform annular coolant flow prior to its exit from the slot into the combustion chamber.
As a result, the annualr film of coolant air tends to persist as it rapidly flows downstream along and against the inner surface of the combustion chamber sidewall while resisting mixing with the inner hot gas flow.
Some structural difficulties have been encountered with the extended lip structure. Thus, tests have demonstrated that the extended cantilevered lip is susceptible to buckling as it attempts to expand radially under thermal stress. When lip buckling has occurred, the annular coolant slot has become partially obstructed by the outward buckling lip portions causing major distortions in the coolant air flow through the slot followed by substantial hot gas mixing with the boundary air layer and a resultant significant or unacceptable loss of downstream cooling effectiveness on the chamber sidewall.
As shown in the aforementioned patent application, slots can be provided in the lip to allow for thermal growth without lip buckling. However, the gain made against buckling is offset to some extent by general weakening of the lip structure and by some resultant encouragement of mixing of the slot coolant air and the hot gas flow through the slots. The slotted lip thus can avoid buckling but it does so at some compromise in lip strength and cooling effectiveness.